Anabela Gregório Dias

Characterization of Geometric Uncertainties of a Dose Calibrator During Measurement of 90Y Activity

The performance and measurement of 90Y with a dose calibrator has always been a key point of several international recommendations, specifically in the case of the radiopharmaceutical 90Y-ibritumomab tiuxetan, among other applications, used in the treatment of non-Hodgkin lymphoma. The objective of the present work was to examine in detail some important sources of geometric errors that, if neglected, can lead to an increase in uncertainty about the measured activity of this β− emitter radionuclide. Methods: A CRC-15R dose calibrator was used to measure and quantify some of these sources, although the same methodology can easily be applied to any other similar equipment. The depth response along the main axis of the dose calibrator was carefully characterized, as well as syringe volume effects and source angular dependence. Results: It was found that, if not taken properly into account, these issues can contribute to an increase in the activity uncertainty (e.g., 5.1% in the present example). This finding implies the possibility of easily reaching higher values than the internationally suggested 5% uncertainty in activity measurement for therapeutic purposes. Conclusion: These errors can be greatly reduced by previous characterization of the dose calibrator and careful implementation of the methodology for measurement.

Effects of shielding on pelvic and abdominal IORT dose distributions

PURPOSE To study the impact of shielding elements in the proximity of Intra-Operative Radiation Therapy (IORT) irradiation fields, and to generate graphical and quantitative information to assist radiation oncologists in the design of optimal shielding during pelvic and abdominal IORT. METHOD An IORT system was modeled with BEAMnrc and EGS++ Monte Carlo codes. The model was validated in reference conditions by gamma index analysis against an experimental data set of different beam energies, applicator diameters, and bevel angles. The reliability of the IORT model was further tested considering shielding layers inserted in the radiation beam. Further simulations were performed introducing a bone-like layer embedded in the water phantom. The dose distributions were calculated as 3D dose maps. RESULTS The analysis of the resulting 2D dose maps parallel to the clinical axis shows that the bevel angle of the applicator and its position relative to the shielding have a major influence on the dose distribution. When insufficient shielding is used, a hotspot nearby the shield appears near the surface. At greater depths, lateral scatter limits the dose reduction attainable with shielding, although the presence of bone-like structures in the phantom reduces the impact of this effect. CONCLUSIONS Dose distributions in shielded IORT procedures are affected by distinct contributions when considering the regions near the shielding and deeper in tissue: insufficient shielding may lead to residual dose and hotspots, and the scattering effects may enlarge the beam in depth. These effects must be carefully considered when planning an IORT treatment with shielding.

Gafchromic XR-QA2 film as a complementary dosimeter for hand-monitoring in CTF-guided biopsies

Computed tomography fluoroscopy (CTF) is a useful imaging technique to guide biopsies, particularly lung biopsies, but it also has the potential for very high hand exposures, despite use of quick-check method and needle holders whenever feasible. Therefore, reliable monitoring is crucial to ensure the safe use of CTF. This is a challenge, because ring dosimeters monitor exposure only at the base of one finger, while the fingertips may be exposed to the highly collimated CT beam. In this work we have explored the possibility of using Gafchromic XR-QA2 self-developing film as a complementary dosimeter to quantify hand exposure during CTF-guided biopsies. A glove used in a previous study and designed to contain 11 TLDs was adapted to include Gafchromic strips 7 mm wide, covering the fingers. A total of 22 biopsies were successfully performed wearing this GafTLD glove under sterile gloves, and the IR reported no difficulty or reduction of dexterity while wearing it. Comparison of dose distributions obtained from digitization of the Gafchromic film strips and absolute Hp(0.07) readings from TLDs showed good agreement, despite some positional uncertainty due to relative movement. Per procedure, doses at the base of the ring finger can be as low as 3%-8% of hand dose maximum. Accumulated dose at the base of the ring finger was four times lower than the dose maximum. PACS numbers: 07.57.Kp, 29.40.-n, 85.25.Pb, 87.57.qp.Computed tomography fluoroscopy (CTF) is a useful imaging technique to guide biopsies, particularly lung biopsies, but it also has the potential for very high hand exposures, despite use of quick‐check method and needle holders whenever feasible. Therefore, reliable monitoring is crucial to ensure the safe use of CTF. This is a challenge, because ring dosimeters monitor exposure only at the base of one finger, while the fingertips may be exposed to the highly collimated CT beam. In this work we have explored the possibility of using Gafchromic XR‐QA2 self‐developing film as a complementary dosimeter to quantify hand exposure during CTF‐guided biopsies. A glove used in a previous study and designed to contain 11 TLDs was adapted to include Gafchromic strips 7 mm wide, covering the fingers. A total of 22 biopsies were successfully performed wearing this GafTLD glove under sterile gloves, and the IR reported no difficulty or reduction of dexterity while wearing it. Comparison of dose distributions obtained from digitization of the Gafchromic film strips and absolute Hp(0.07) readings from TLDs showed good agreement, despite some positional uncertainty due to relative movement. Per procedure, doses at the base of the ring finger can be as low as 3%–8% of hand dose maximum. Accumulated dose at the base of the ring finger was four times lower than the dose maximum. PACS numbers: 07.57.Kp, 29.40.‐n, 85.25.Pb, 87.57.qp

Attenuation measurements show that the presence of a TachoSil surgical patch will not compromise target irradiation in intra-operative electron radiation therapy or high-dose-rate brachytherapy

BackgroundSurgery of locally advanced and/or recurrent rectal cancer can be complemented with intra-operative electron radiation therapy (IOERT) to deliver a single dose of radiation directly to the unresectable margins, while sparing nearby sensitive organs/structures. Haemorrhages may occur and can affect the dose distribution, leading to an incorrect target irradiation. The TachoSil (TS) surgical patch, when activated, creates a fibrin clot at the surgical site to achieve haemostasis. The aim of this work was to determine the effect of TS on the dose distribution, and ascertain whether it could be used in combination with IOERT. This characterization was extended to include high dose rate (HDR) intraoperative brachytherapy, which is sometimes used at other institutions instead of IOERT.MethodsCT images of the TS patch were acquired for initial characterization. Dosimetric measurements were performed in a water tank phantom, using a conventional LINAC with a hard-docking system of cylindrical applicators. Percentage Depth Dose (PDD) curves were obtained, and measurements made at the depth of dose maximum for the three clinically used electron energies (6, 9 and 12MeV), first without any attenuator and then with the activated patch of TS completely covering the tip of the IOERT applicator. For HDR brachytherapy, a measurement setup was improvised using a solid water phantom and a Farmer ionization chamber.ResultsOur measurements show that the attenuation of a TachoSil patch is negligible, both for high energy electron beams (6 to 12MeV), and for a HDR 192Ir brachytherapy source. Our results cannot be extrapolated to lower beam energies such as 50 kVp X-rays, which are sometimes used for breast IORT.ConclusionThe TachoSil surgical patch can be used in IORT procedures using 6MeV electron energies or higher, or HDR 192Ir brachytherapy.

The use of needle holders in CTF guided biopsies as a dose reduction tool

Abstract Purpose The purpose of this study was to evaluate the efficacy of needle holders in reducing staff hand exposure during biopsies guided by computed tomography fluoroscopy (CTF), through the analysis of data acquired during a detailed monitoring study, undertaken in parallel with an ongoing optimization process to reduce hand irradiation. Methods Hand monitoring was performed with 11 extremity detectors, two per finger (base and tip) and one on the back of the wrist, for the left (dominant) hand, during two series of biopsies with comparable characteristics. The first series (47 biopsies) were performed with only quick‐check method (QC) and occasional side‐handle (SH) manipulation of the needle. The second series (63 biopsies) were performed after introducing needle holders (NH) in the course of an optimization process. Results Choice of technique (QC, QC + NH, QC + SH) by the interventional radiologist (IR) was related to biopsy difficulty. Measured hand exposure was low (< 1 mSv) for all QC‐only procedures, and for most of the QC + NH procedures. Occasional side‐handle manipulation still occurred during challenging biopsies, so that 8% of biopsies in the second series accounted for ~70% of total fingertip dose (~90 mSv). The methodology used allowed a detailed insight into the dose reduction achievable with needle holders during real procedures, without the limitations of phantom measurements. Conclusions Needle holders proved effective in reducing mean hand exposure during clinical procedures where real‐time manipulation was necessary. Occasional side‐handle manipulation was found to contribute disproportionately to hand exposure. This highlights the importance of individual hand monitoring during CTF guided procedures.

Prostate SBRT: The use of ERB and MOSFET in vivo dosimetry feasibility

Introduction Hypofractionated treatment regimens are indicated for some stages of prostate cancer. In order to provide the reproducibility of the relative position between rectum and prostate and to allow that only a small volume of the rectal wall remains close to the prostate, an endorectal balloon (ERB) may be used. Purpose This work aims to assess the dose on the rectal wall in the presence of the ERB filled with water or air and evaluate the deviation between the measured and calculated doses using two different algorithms (Eclipse AAA and iPlan PencilBeam) Materials and methods Two CT scans were obtained for a modified Rando phantom where an ERB (filled with water or air) with three MOSFET detectors was inserted. A simple 4 field in box plan and an IMRT (6MV beam) clinical plan were calculated in both CT sets, using two different algorithms. The treatment plans were delivered to the phantom using a Varian Novalis linac. Four sets of measurements were obtained and the results were compared with the calculated values. Results The difference between the calculated and measured doses around the ERB is lower when it is filled with water for both algorithms. The maximum relative differences when the ERB is filled with air are 1.8% for Eclipse and 5.5% for iPlan. Conclusion Both algorithms show a better a performance in the presence of water. Water filled ERB seems to be a suitable option. MOSFET dosimetry in association with ERBs for real-time in vivo during hypofractionated treatment of the prostate is a practical and reliable procedure. Disclosure The authors have no relevant financial or non-financial relationships to disclose.

Low Cost Alternative to Lead Glass Shielding in PET/CT Control/Scanner Room Window

The aim of this work is to propose a reasonably priced alternative to expensive lead glass used in PET/CT windows connecting the control room and the scanner room. These windows can be extremely useful to detect if the patient needs quickly to be assisted, for anesthesia surveillance when needed, among several other purposes and is mandatory to exist in these facilities. The window must provide also radiation protection to the worker inside the control room. One way to accomplish this task is to use lead glass. However, lead glass is expensive and is usually manufactured as predetermined lead equivalent thicknesses (frequently 2 mm or more). It is also designed to be lead equivalent with nominal thicknesses specified for radiology energy ranges. Depending on the distances involved and on the workload, since the photon energy irradiating from PET patients is significantly higher (511 keV) than other nuclear medicine and radiology procedures, it is usually necessary to protect the professionally exposed worker with a high thickness of lead or lead equivalent glass. For a big area window, a lead glass plate can be quite expensive. In this work it is presented a less expensive solution using common glass with thickness higher than 15 cm. Calculations are presented for typical situations and workloads, assuring protection both from PET and CT.

Philips Astonish image processing software phantom evaluation used on planar mode acquisition images

A multi-phantom study is presented, comparing planar image properties modification while using Philips Astonish image enhancement software. Three types of phantoms were used to evaluate the effects of the software on the images and to estimate when and how this software can be used safely without introducing relevant artifacts that could induce clinical misreading and wrong diagnosis. According to the manufacturer, the point spread function (PSF) problem, which is mandatory for precise image deconvolution on an Anger camera (or gamma-camera), was solved and a corrected object image estimation can be achieved from the blurred original acquired image. Regardless the apparent clinical planar image enhancement achieved with the Philips Astonish software, it was found that, in a phantom planar acquisition image evaluation, the inexistence of a precise PSF description leads, in some cases, to a notorious inaccurate image reconstruction. The obtained experimental and reconstructed results were compared with an image deconvolution algorithm, where the PSF is a known parameter. Thus, the difficulty of not knowing exactly this parameter in an Anger camera still leads to the failure of exact activity quantification, as well as precise morphologic determination of the isotope distribution inside the body, when exact location of this distribution is not known in advance. Nevertheless, the Philips Astonish software can be helpful in planar imaging when used with precaution and as a tool for a better clinical image presentation when contrast enhancement is desired and when a careful clinical image evaluation is previously carried out.